In papermaking processes, fibrous pulp is slurried in water and various particulate materials are added to improve the quality of the paper product. Such materials are often fillers, for example clays, starch, calcium carbonate, and the like. Such fillers and small cellulose fibers which tend to separate from the bulk of the paper fiber are referred to generically as fines.
It is important in papermaking to obtain good retention of fines in the paper product. Good fines retention helps to achieve better and more consistent final dry paper properties and permits a more efficient and cost effective usage of the pulp stock. When the retention of fines in the product is high, less pulp is used to produce the final product and the process water is much cleaner.
Fines retention becomes even more important when paper is made from recycled waste because pulp from recycled papers has a higher level of fines, under normal conditions, than the pulp used to make the original product. When these fines are not retained on the paper, cost effective usage of the recycle stock declines and the higher fines levels can lead to more frequent and costlier cleanup of the processing water.
A common method for retaining these fine particles is to add alum which negates the repulsive forces between the negatively charged cellulosic surfaces and the negatively charged filler particles. Following this, a cationic polymer is added which bridges the two types of anionic surfaces and binds them together. Presently, several types of polymers along with varying methods are used to help improve fines retention. Such polymeric types include cationic polymers such as copolymers of acrylamides and quaternary amines, anionic polymers such as copolymers of acrylamide and acrylic acid, and amphoteric polymers such as a quaternary amine and acrylic acid. Several newer systems which are now being used include blends of cationic polyacrylamides with anionic fillers, such as kaolin clays. Another type of fines retention aid is a blend of cationic starch with anionic colloidal silica.
U.S. Pat. No. 3,840,489, Strazdins (1974) discloses improving the dry strength of a paper product by adding to the pulp in the papermaking process an aqueous dispersion of a copolymer of acrylamide and a hydrophobic vinyl comonomer, such that the ratio of acrylamide linkages to hydrophobic linkages is between 60:40 and 95:5. The hydrophobic linkages are said to improve the adsorptivity of the polymer by cellulose fibers.
Amine functional polymers are known to be valuable as a cost effective way of incorporating cationic charges into the polymers. Such polymers have found utility in cationic electrocoating, water treatment and enhanced oil recovery.
U.S. Pat. No. 4,843,118, Lai, et al. (1989) discloses the use of high molecular weight (greater than 1.times.10.sup.6) poly(vinylamines) in acidized fracturing fluids for enhanced oil recovery. Such poly(vinylamines) can be prepared by acid or base hydrolysis of poly(N-vinylformamide). Although these high molecular weight poly(vinylamines) can be used in enhanced oil recovery without crosslinking, the use of crosslinking agents, such as epichlorohydrin, is disclosed as optional. The use of dialdehyde, such as glyoxal, to crosslink poly(vinylamine) is also disclosed in Japanese Patent Publication No. J61051006 (1986).
U.S. Pat. No. 4,421,602, Brunnmueller et al. (1983) discloses partially hydrolyzed homopolymer of N-vinylformamide useful as a drainage aid in papermaking. From 10 to 90% of the formyl groups are hydrolyzed to amine units in either acid or base in making this homopolymer.
U.S. Pat. No. 4,808,683, Itagaki et al. (1989) discloses a vinylamine copolymer of N-vinylformamide and an alkyl or oxoalkyl N-substituted acrylamide or methacrylamide in which the formamide units have been partially hydrolyzed under acidic conditions to cationic amine units. The copolymer is said to be useful as a flocculating agent and in papermaking as a drainage aid and to increase paper strength.
Despite such wide variety of retention aids, there continues to be a need for even better fines retention agents as the use of recycled papers grows. Indeed, the economics of recycled paper has become an important environmental issue.